The Falcon 5 was a proposed two-stage-to-orbit partially reusable launch vehicle designed by Space Exploration Technologies Corp., doing business as SpaceX, since canceled
in preference of the larger, more powerful Falcon 9.[1]

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The first stage of Falcon 5 was to be powered by five Merlin engines and the upper stage by one Merlin engine, both burning RP-1 with a liquid oxygen oxidizer. Along with the Falcon 9, it would have been the world's only launch vehicle with all stages designed for reuse.[2]

The Falcon 5 would have been the first American rocket since the Saturn V to have full engine out capability, meaning that with the loss of one engine, it can still meet mission requirements by burning the other four engines longer to achieve the correct orbit.[3] In comparison, the Space Shuttle only had partial engine out capability, meaning it may not be able to achieve proper orbit by burning the remaining engines longer.[3]

Although an original Falcon 5 was never built, in December 2011 Stratolaunch Systems announced that they planned to develop a four- or five-engine Falcon 9-derivative two-stage liquid-fueled air-launched launch vehicle to be developed by SpaceX.[9]
The launch vehicle was planned to be "along the lines of the company's Falcon 4 [sic] or Falcon 5,"[10]
and was to have been capable of launching a 6,100 kilograms (13,500 lb) payload into low earth orbit.[11]

The air-launched rocket concept was eventually named the Falcon 9 Air, and was being designed to have only four Merlin 1D engines. However, development was halted in late 2012 when SpaceX and Stratolaunch "amicably agreed to end [their] contractual relationship because the [Stratolaunch] launch vehicle design [had] departed significantly from the Falcon derivative vehicle envisioned by SpaceX and does not fit well with [SpaceX's] long-term strategic business model."[12]